Objectives To demonstrate the process of radicalisation within the prison environment, via the notion of radicalogenesis, by analogy with the concept of criminogenesis While the latter focuses on the pre-criminal situation and studies what prompts a subject to engage in criminal activity, for the genesis of radicalisation we concentrate on the post-criminal situation of individuals, in order to untangle the stages that can bring a prisoner to become radicalised. Method An analysis of the literature on the process of radicalisation, and on the genesis of criminality; this is backed up by the methodology of clinical follow-up in the context of the psychological/psychotherapeutic work undertaken with detainees over a period of six years in French prisons. Results The results from the follow-up of detainees caught up in a process of radicalisation, and from the review of the literature show that the prison environment can facilitate radicalisation through a process whereby the identity of an individual becomes more vulnerable following a criminal act that leads to incarceration – an act that is sometimes traumatic for the perpetrator. Discussion Having summarised, in table form, the different theories of radicalisation, and discussed their points of similarity/difference, we describe four phases in the genesis of radicalisation: vulnerable identity, dis-identification, conversion and re-identification, radicalisation. We want to emphasise that this process is strictly individual and depends on the subjective structure of the person becoming radicalised. Conclusions We reiterate the importance of psychological/psychotherapeutic support in prisons, particularly for the more vulnerable individuals, and those made more vulnerable still by their acting-out. Without this support, the individual risks becoming radicalised in order to find a new pole of identification, as a means of survival.